fix(desktop): unblock note sharing

[ComputelessComputer]

Scope share preparation writes to the active note and cache, and resize the share popover to 320x240.

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Note

Medium Risk
Keyed flush changes persistence timing for sharing and could affect race behavior if callers pass wrong keys; the new email endpoint touches auth and third-party email delivery.

Overview
Unblocks desktop note sharing by letting flushDatabaseWrites target specific queue keys (e.g. session:{id}) so share preparation waits only on that note’s writes, not unrelated background sync. Per-key flush also drains newer writes on the same key before surfacing an earlier failure, with tests covering both behaviors. Share flows call the keyed flush when loading share source and flushing canonical editor changes.

Adds server support for invitation emails: POST /shared-notes/invitations/{invitation_id}/email (OpenAPI + SharedNoteInvitationEmailRequest) and wires SharedNotesConfig with Loops via with_invitation_email in API startup (api-sync gains loops dependency).

Refreshes session-sharing i18n—drops older copy (e.g. copy-invite, long conflict instructions) in favor of shorter labels for invite, access levels (full / invited-only), restrict, and share.

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This is part 1 of 2 in a stack made with GitButler:

•  2  Keep local interactions responsive #6204
•  1  fix(desktop): unblock note sharing #6202 👈

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