feat: shortcuts for every screen-level control

[ttncode]

Summary

Every control that appears once per screen now has a shortcut (rebindable in Ctrl+/, hinted on hover). Row-level buttons in lists stay reachable with Tab + Enter.

Screen	Control	Default
Focus	Reset session	Alt+R
Focus	Extend finished session	Alt+Shift+E
Focus	Edit minutes	Alt+I
Focus	Auto-shuffle animations on/off	Alt+Shift+A
Focus	Next animation	Alt+Shift+C
To-Dos / Habits / Goals / Tags	Show / hide archived	Alt+Shift+H
To-Dos	Show / hide backlog	Alt+Shift+K
Habits	Switch 7 / 30 days	Alt+Shift+D
Habits, Focus summary	Previous / next period	Alt+Shift+← / →
Focus summary	Next chart range (7→30→90→180→365 days)	Alt+Shift+P
Boost	Public / private post	Alt+Shift+V
Tags	Back	Alt+Shift+B
About Me	Save	Ctrl+S
Menu	Sounds / Notifications	Alt+Shift+S / Alt+Shift+O

Defaults avoid keys browsers keep for themselves (Alt+←/→ back/forward, Alt+D/E/F). Each handler is enabled only while its control is on screen. Ctrl+S is captured on About Me only, and saves only when there are unsaved edits. The Notifications logic is shared between the menu item and the shortcut. README shortcut table updated.

Testing

• Lint, typecheck, unit tests and build pass.
• Every new shortcut exercised in the browser at 1440x900 with its visible effect confirmed (reset/extend states, minutes focus, shuffle flag, animation cycling, archived/backlog/range toggles, period navigation incl. disabled forward arrow, chart range cycle, visibility toggle, back, save, Sounds panel, Notifications dialog). All 16 appear in the Ctrl+/ panel.
• Not reached: the Notifications "ask permission" branch (test browser had permission denied); it reuses the menu item's existing logic.