fix(web): shift jump, focus contrast, and recurrence nudge bugs

[tyler-dane]

Summary

Fixes four calendar interaction bugs:

1. Keyboard-only + Shift jump — Shift event-jump chips/labels now work while keyboard-only mode is on (the previous guard blocked activate()).
2. Light-mode focus contrast — Event cards use ring-offset-background on focus, and timed selected state uses a dual-tone halo so default charcoal fills stay distinguishable.
3. Recurrence toast 1/2 after SHIFT+ARROW — Local replaceSeries now applies the occurrence edit as a delta onto the series base (instead of rebasing DTSTART) and deletes leftover occurrence overrides so subsequent instances stick after settle.
4. Undo after recurring SHIFT+ARROW — Local preserve on a composed occurrence id keeps { kind: "occurrence" } linkage (mirroring backend ack) so undo restore no longer demotes to single. Undo/redo restores also skip write coalescing.

Test plan

• bun test:web focused on shift-hint, keyboard-only integration, EventCard, local.event.repository, useEventMutations, useUndoRedo
• bun lint (no new errors)
• Manual: keyboard-only → Shift shows jump chips
• Manual: light theme focus ring on default-color event
• Manual: all-day recurring SHIFT+ARROW → toast 1/2 moves siblings
• Manual: same nudge → undo with no catchall toast