Persist paste pull cursor for discarded oversized remote pastes

[guiyanakuang]

Closes #4702

Problem

#4699 discards oversized remote non-file pastes without storing them, but the pull cursor only advances in memory. After a restart, PastePullService.init() rebuilds each device's cursor from the max createTime of stored pastes — which never includes a discarded paste. If the newest paste from a peer was discarded, every restart re-pulls it, re-discards it, and re-notifies, repeatedly transferring tens of MB until the peer produces a newer stored paste.

Changes

• New PastePullCursorEntity table (appInstanceId PK, maxCreateTime) with SQLDelight migration 3.sqm. The migration uses CREATE TABLE IF NOT EXISTS so it stays idempotent under DesktopDriverFactory's migration replay after a fresh create.
• Discard path (PasteReleaseService.discardOversizedRemoteNonFilePaste) upserts the cursor with the discarded paste's createTime; the SQL upsert takes MAX(existing, excluded) so the cursor never moves backwards.
• PastePullService.init() restores each device's cursor as the max of the stored-paste max createTime and the durable cursor.
• Device removal cleanup: SyncDeviceManager.removeDevice (reached by both local unpair and remote NOTIFY_REMOVE) now deletes the device's cursor row alongside the other local state. PasteDao is injected directly rather than going through PastePullService to avoid a constructor dependency cycle via SyncManager.

Safety notes

• The durable cursor is written synchronously in the discard path; in the pull batch flow items are processed in ascending createTime order and persisted inline, so the cursor can never cover an older item that has not yet been stored.
• The pull query uses strict createTime > :createTime, so a cursor equal to the discarded paste's createTime is sufficient to suppress re-pulls.

Tests

• PasteDaoTest: cursor upsert is monotonic; delete only removes the given device's row.
• PasteReleaseServiceRemoteDiscardTest: discarding upserts the durable cursor exactly once.
• PastePullServiceTest (new): init() merges stored max and durable cursor per device; in-memory updates never write the durable cursor.
• SyncDeviceManagerTest: removeDevice deletes the cursor row.

Note for mobile: shared/commonMain changes include the new table + 3.sqm; the mobile repos must pick up the migration file when syncing shared code.