feat(companion): live location sharing, Locator geofencing, trail auto-pause

Squash merge of feat/location-beacon-alerts-autopause (v1.21).

Features:
- Live Location Sharing — broadcast position as movement-gated [LOC]
  messages to a channel or contact; live shares show as map pins with
  distance/bearing in Nearby Nodes and a status-bar indicator. [LOC] is
  parsed in DMs, channel messages and room messages; DM shares name the
  sender.
- Locator (geofence) — arm a geofence around a saved waypoint or a person
  (live [LOC] or last-known position), alert on arrive/leave or near/far,
  with an optional homing beeper (gated to arrive/both modes). Arm from
  Tools > Locator, Nearby Nodes, or Waypoints; target picker lists
  favourites first, clearable via a "None" entry. Active target is drawn
  as a flag on the map.
- One active target shared across Locator / Navigate / Map via a single
  resolver (resolvePersonPos / activeTargetPos) that prefers a live
  [LOC] share over the last-advertised GPS fix.
- Follow live contacts — Navigate to a live-sharing contact follows them
  as they move and adds an ETA line; quick-share your own position from
  the Map.
- Map & status-bar upgrades — home mini-map gets a north marker, scale
  tick, and a connected trail line (was disconnected dots); status line
  shows tracked-node count, an arrow + distance to the active Locator/Nav
  target (falling back to the nearest live-tracked contact); GPS fix icon
  in the top status bar, shown only on GPS boards with GPS enabled.
- Trail auto-pause — recording freezes on stops (banking elapsed time,
  breaking the map line across the idle gap) and resumes on movement
  without ending the session.
- Streaming trail simplification — GPS points are simplified in-stream
  via a fixed-corridor (Reumann-Witkam) pass tuned for fidelity: straight
  runs collapse to their endpoints, curves stay bounded to the Min-dist
  tolerance, so the 512-point buffer covers a far longer route than a
  flat point budget would suggest.
- Collapsible Tools (Location / Comms / System sections, fold-in-place
  like Settings) and page-indicator icons on the home carousel.
- Waypoint coordinate editor — add a waypoint by scroll-editing lat/lon
  digit by digit.

Fixes:
- Critical: low-heap hang and contact loss on RAM-tight builds. Halved
  message-history scrollback rings (recovering ~14 KB free heap) and
  made contacts/channels/prefs persistence atomic (temp file + rename),
  so an interrupted save can no longer corrupt or wipe the store.
- Serial.write() bounded so a stalled USB host can't hang the device.
- Nearby Nodes: live [LOC] senders respect the type filter, sort by
  shared position, and the list refreshes so live shares bubble up.
- Map: live contacts are labelled before waypoints.
- GPS status icon hidden when GPS is off in Settings.
- Splash screen no longer truncates a pre-release tag's own dash (e.g.
  v1.21-rc1) when stripping the build's commit-hash suffix.
- Null-guarded the Locator target picker; clamped loc-share channel
  index on load.

Under the hood:
- -Os size optimisation on the e-ink and GAT562 30S solo envs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -36,7 +36,9 @@ All firmware files are published on the [releases page](https://github.com/Marek
   - **GPS compass** — heading derived from course-over-ground (no magnetometer needed), shown as a clear scrolling heading tape with a large degrees + cardinal readout
   - **Navigate to anything** — a saved waypoint, a node straight from Nearby Nodes, or a location someone shares with you in a message
   - **Share & save locations** — send a waypoint to a contact or channel; on the other end, navigate to or save any shared location with one menu
-  - **GPS trail** — background route recording with an auto-fit map (waypoints + live position), summary stats, and [GPX export](#solo-tools)
+  - **Live location sharing** — broadcast your position over the mesh as you move (movement-gated, to a channel or contact) and see others who share theirs as pins on the map and live distance/bearing in Nearby
+  - **Locator** — arm a geofence around a target — a saved waypoint *or* a person (their live/last-known position) — and get an alert when you arrive/leave or they get near/far, with an optional homing beeper that ticks faster the closer you get. Set it from the Locator screen or straight from Nearby Nodes / Waypoints, and see the target as a flag on the map
+  - **GPS trail** — background route recording with an auto-fit map (waypoints + live position), summary stats, auto-pause on stops, and [GPX export](#solo-tools)
   - **Metric or imperial** — one global Units setting drives every distance and speed across the UI
 
 - [Messages Screen](./docs/solo_features/message_screen/message_screen.md) — view and send messages, open message details, reply with quick messages or custom text, navigate to / save locations shared in a message, per-channel notification and melody overrides
@@ -49,7 +51,7 @@ All firmware files are published on the [releases page](https://github.com/Marek
 
 - [Screen Lock](./docs/solo_features/screen_lock/screen_lock.md) — lock the device to prevent accidental keypresses, with a lock screen showing time and sensor data
 
-- [Tools Screen](./docs/solo_features/tools_screen/tools_screen.md) — GPS trail & waypoints, compass, nearby nodes (with ping & navigate), ringtone editor, auto-reply bot, auto-advert, diagnostics, repeater
+- [Tools Screen](./docs/solo_features/tools_screen/tools_screen.md) — GPS trail & waypoints, compass, nearby nodes (with ping & navigate), ringtone editor, auto-reply bot, auto-advert, live location sharing, locator, diagnostics, repeater
 
 - **Battery saving (radio)** — two optional, independent toggles under Settings › Radio:
   - **Pwr save** — hardware duty-cycle receive (SX126x `SetRxDutyCycle`): the radio cycles RX↔sleep on its own and wakes on a preamble, cutting average RX current with only a little added receive latency
@@ -97,7 +99,7 @@ Updating to a newer version usually does not require erasing flash unless the re
 | [Clock Screen](./docs/solo_features/clock_screen/clock_screen.md)          | Clock page, date, configurable data fields                            |
 | [Settings Screen](./docs/solo_features/settings_screen/settings_screen.md) | All settings sections with values and interactions                    |
 | [Screen Lock](./docs/solo_features/screen_lock/screen_lock.md)             | Lock/unlock sequence, lock screen, auto-lock                          |
-| [Tools Screen](./docs/solo_features/tools_screen/tools_screen.md)          | GPS trail & waypoints, compass, navigation, nearby nodes, ringtone editor, auto-reply bot, auto-advert, diagnostics, repeater |
+| [Tools Screen](./docs/solo_features/tools_screen/tools_screen.md)          | GPS trail & waypoints, compass, navigation, nearby nodes, ringtone editor, auto-reply bot, auto-advert, live location sharing, locator, diagnostics, repeater |
 
 ### Upstream MeshCore