A11y safeguard: CI role check, remaining fixes, and coding rules

[mapgie]

Summary

Follow-up to the a11y audit (PR #91). Adds safeguards so accessibility regressions are caught automatically on every PR, and fixes the remaining violations not caught in the first pass.

• a11y_check.py: Python CI script that scans every Compose source file for .clickable / .combinedClickable modifier chains missing a .semantics { role = Role.* } declaration. Fast-fails (no JDK required) so violations are surfaced before a full Gradle build.
• CI step in build.yml: Runs python3 a11y_check.py --fails-only as the first job step on every PR targeting main.
• Additional role fixes (missed in the initial audit):
  • ManageCategoriesScreen.kt archive dialog: "Don't show this again" Row now has Role.Checkbox and inner Checkbox(onClick = null).
  • SettingsScreen.kt reminder settings: alarm-permission ListItem now has Role.Button.
  • SettingsScreen.kt export sub-screen: export-scope rows have Role.RadioButton + RadioButton(onClick = null); export-format rows the same.
  • SettingsScreen.kt theme picker: mode selector Boxes have Role.RadioButton; PaletteOption Column has Role.RadioButton; IconChoiceCell Column has Role.RadioButton.
  • CalendarGrid.kt: role = Role.Button added inside clearAndSetSemantics so day cells are reachable by keyboard and switch access.
  • HomeScreen.kt: speed-dial scrim dismiss overlay now has Role.Button and contentDescription = "Close menu".

Test plan

• Run python3 a11y_check.py locally — should print "All clean" with exit 0.
• Open Settings, theme picker: TalkBack should announce each mode option (Light/Dark/System) as a radio button.
• Open Settings, palette picker: each colour circle should be announced as a radio button with the palette name.
• Open Settings, app-icon picker: each icon tile should be announced as a radio button.
• Open any export dialog in Settings: scope rows (Data only / Full backup) and format rows (JSON/CSV) should each be a single focusable radio button.
• On the home calendar, tap a day with TalkBack active: cell should be announced as a button with its date and status description.
• Open the speed dial on the home screen with TalkBack, then explore-by-touch the scrim: should announce "Close menu, button".
• Open a category, long-press the archive action, check "Don't show this again" with TalkBack: should announce as a checkbox.

https://claude.ai/code/session_01AeVcEqKNsCwEG6iSF4MYt4

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