perf: batch E2EE queries, batch dir rename, hash lookup, larger Windows watcher buffer

[qoole]

Summary

A set of related performance fixes touching sync startup, propagation, and the tray.

Batch E2EE path queries (syncjournaldb.cpp)

getRootE2eFolderRecord and findEncryptedAncestorForRecord previously walked up the path one level at a time, issuing a separate getFileRecord per level. For a path 8 levels deep that's 8 round trips to SQLite on every E2EE-related lookup.

Replaced with a single WHERE path IN (...) query that fetches all candidate ancestors at once. The result is then walked locally to find the deepest e2e-encrypted match.

Batch directory rename (propagatorjobs.cpp)

PropagateLocalRename::start previously processed child journal entries inline while iterating a streaming SELECT. Mixing reads and writes on the same SQLite statement is fragile and prevented batching. Refactored to:

1. Collect all child records into a vector while the SELECT is open.
2. Close the read.
3. Apply the rename mutations in a single pass.

This makes the operation safer and lets the journal batch its writes.

Remove Instantiator destroy/recreate hack (MainWindow.qml, CurrentAccountHeaderButton.qml)

The tray was destroying and recreating its account Instantiator on every menu open/close as a workaround for an old binding glitch. This causes a full delegate teardown and rebuild every time the user opens the tray. The hack is no longer needed; removed it.

O(n²) -> O(1) folder adjustment (owncloudpropagator.cpp)

adjustDeletedFoldersWithNewChildren looked up each item with std::find_if over the full vector, making the function O(n²). Replaced with a QHash<QString, int> index built once and reused for each lookup.

Windows file watcher buffer (folderwatcher_win.cpp)

The ReadDirectoryChangesW buffer was 40KB initial / 64KB max, which overflows easily under heavy file churn (e.g. large extracts, IDE save cascades). On overflow Windows reports a single "buffer overflow" event and the watcher falls back to a full-tree rescan, which is extremely expensive on large folders.

Bumped to 256KB initial / 2MB max. The buffer only grows as needed.

Checklist

• Sign-off message is added to all commits.
• The commit history is clean with no merge commits.
• Uploaded screenshots from before and after for UI changes (tray Instantiator change is not visually different to the user; account list still populates as before).
• Documentation has been updated or is not required.
• Backports requested where applicable (critical bugfixes).
• Labels added where applicable (bug/enhancement).
• Milestone added for target version.

AI (if applicable)

• The content of this PR was partly or fully generated using AI