fix(app): raise the window a repository already has

[LeadcodeDev]

Closes #45.

gitr <path> for a repository already open added a second window for it. The handoff was
never the problem — it routes the path correctly — and ProjectList::add_or_activate
dedupes the persisted list, which has nothing to do with what is on screen.

The key is already canonical

A HashMap<PathBuf, WindowHandle<Root>>, consulted before opening. Both sides agree on the
key without any new normalisation: resolve_repository_root canonicalises before the path
goes into the socket, which is what makes /tmp/x and /private/tmp/x one entry rather
than two. The transcript below shows that happening.

No window-close hook

The issue's sketch says "populated when open_window succeeds and cleared when that window
closes". The second half is not needed and is the riskier half.

WindowHandle::update returns Err once its window is gone, so raising is the liveness
check: a closed window's entry is detected and dropped the next time that repository is
asked for, and the request that found it stale is the one that opens the replacement.
Pruning eagerly would buy back one map entry per repository opened in this process. That
leak is worth less than the code that plugs it — and a close hook that missed a case would
fail in the worse direction, leaving an entry that outlives its window and stops the
repository ever opening again.

Verified on the real handoff

No UI automation involved — these are four command-line invocations against one running
instance, with the decision logged at each request:

gitr <this repo>     → raised existing window for /Users/…/gitr
gitr /tmp/other-repo → opening new window for   /private/tmp/other-repo
gitr <this repo>     → raised existing window for /Users/…/gitr

The /private/tmp line is the canonicalisation doing its job.

Not exercised: the stale-handle path. Closing a window needs the UI, which this cannot
drive. What needs a human: close a repository's window, then run gitr for it again — it
should open fresh rather than do nothing.

cargo test --workspace green; cargo clippy --workspace --all-targets -- -D warnings;
cargo fmt --all --check.