Guard the pledge share clipboard fallback

[posthog]

Problem

• The pledge "Share this pledge" button could fail with no feedback, on the share loop that drives new pledges before the October 26 vote. The user saw the button stay on "Share this pledge" and never learned the link did not copy.
• After the native share sheet is dismissed, the handler fell through to a bare navigator.clipboard.writeText. The browser rejects that write, because the transient user activation is already gone (NotAllowedError).
• The rejection surfaced as an unhandled rejection, so setCopied(true) never ran — the "Link copied" state never showed.
• The same unguarded handler sat in four copies: Toronto, Brampton, Ottawa, and Hamilton.

Changes

• Add a shared usePledgeShare hook that reports failure instead of failing silently:
  • a dismissed share sheet (AbortError) is treated as done, and the clipboard fallback is skipped — the write would only reject anyway.
  • the clipboard write is wrapped in try/catch and shows a toast on failure, the same pattern already used in ShareButtons.tsx.
• Replace the four copied share handlers with the one hook. Net 64 fewer lines.

Test plan

• pnpm run lint — passes.
• pnpm exec tsc --noEmit — no new type errors (the pre-existing *-stamp.png module errors are unrelated and also hit untouched files).
• Manual: dismiss the share sheet and confirm the button no longer throws; deny clipboard access and confirm a toast appears.

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