task: Location as a top-level settings category, sole home for GPS (#975) #977

Parent: #975

Make Location a top-level Settings category and the only place GPS lives.

Build

• New SettingsCategory directly under Radio Settings: icon Icons.location_on_outlined (the icon the existing Location row already uses), title Location, subtitle "GPS, coordinates, and reporting".
• Its pane, per the approved mockups (cards 1, 2 and 4): a GPS card holding the GPS control and one status row with Query, then a card with Coordinates and Position reporting interval as ListTiles that open their editors.
• Until task: expose GPS mode and Power-save interval -- BLOCKED on firmware 0xC7 (original premise was wrong) #969 lands, the GPS control is the existing on/off, writing the existing gps custom var exactly as today. task: expose GPS mode and Power-save interval -- BLOCKED on firmware 0xC7 (original premise was wrong) #969 swaps it for the three-way control without moving anything.
• Remove the duplicates: the GPS enable SwitchListTile in Node Settings (settings_screen.dart:434-456) and the Location dialog's GPS block (:1415-1466). The Node Settings Location row goes too.
• The existing 0xC1 GPS query section (_buildGpsQuerySection) moves into the status row; capability gating on supportsOffbandGps is unchanged.

Must not change

• What is written to the radio, and when. This is a move, not a behavior change: the same custom vars, the same validation on the reporting interval, the same error paths.
• Radio stats stays where it is.

Done when

• GPS can be reached and changed from Location only; searching the settings tree finds no second GPS toggle.
• A radio with no gps custom var shows the Location category without GPS controls, as the dialog does today.
• flutter analyze and flutter test clean; existing location and GPS tests updated, not deleted.

Filed by MistyLynx (session 6c0d5797).