feat: persist window, splitter and column widths

Resizing the window, the splitter or a thread-list column was undone by
the next launch. State now round-trips through a separate settings file
at ~/.local/state/qtmaildir/uistate.conf, written on close and read at
startup.

The state file is deliberately not the user's config: a base64 geometry
blob does not belong in a hand-edited file, and rewriting that file on
exit would drop its comments and key order, which QSettings does not
preserve.

Two details that are easy to get wrong:

QStandardPaths::StateLocation appends both the organization and the
application name, and both are "qtmaildir" here, so it resolves to
~/.local/state/qtmaildir/qtmaildir. The path is built from
GenericStateLocation instead, matching Config::defaultPath().

Restore runs after buildMenus() rather than at the end of buildUi():
QMainWindow::restoreState() matches toolbars by object name and silently
drops the position of one that does not exist yet.

Every restore is conditional on a non-empty blob, so a missing or
rejected state file leaves the built-in defaults instead of producing a
zero-size window.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 19 insertions, 1 deletions

diff --git a/docs/superpowers/plans/2026-08-03-post-0.1.0-usability.md b/docs/superpowers/plans/2026-08-03-post-0.1.0-usability.md
index 7a6ecc2..debf79a 100644
--- a/docs/superpowers/plans/2026-08-03-post-0.1.0-usability.md
+++ b/docs/superpowers/plans/2026-08-03-post-0.1.0-usability.md
@@ -34,7 +34,7 @@ taking that too literally.
 
 | # | Item | Cluster | Size | Status |
 |---|------|---------|------|--------|
-| 1 | Splitter/column widths do not survive restart | persistence | S | open |
+| 1 | Splitter/column widths do not survive restart | persistence | S | **done** |
 | 2 | No way to see full message details (From/To/Cc/Subject) | information | M | open |
 | 3 | Too few clickable affordances, shortcuts are the only route | discoverability | M | **done** |
 | 4 | Message-pane font size does not survive restart | persistence | S | open |
@@ -89,6 +89,24 @@ as well. Establish it once, in whichever lands first.
 confirm both held. Then delete the state file and confirm the app still starts
 with the 1200x800 default rather than a zero-size window.
 
+### Outcome (done)
+
+Built as described. `MainWindow::uiStatePath()` establishes the state file the
+plan calls for, so items 4 and 10 inherit it. Two things worth recording:
+
+- **`QStandardPaths::StateLocation` is the wrong enum here.** It appends both
+  the organization and the application name, and this app sets both to
+  `qtmaildir`, so it yields `~/.local/state/qtmaildir/qtmaildir/`. The path is
+  built from `GenericStateLocation` plus an explicit `/qtmaildir`, the same
+  shape as `Config::defaultPath()`. A test pins the component count.
+- **`restoreUiState()` runs after `buildMenus()`, not at the end of
+  `buildUi()`** as the plan proposed. `QMainWindow::restoreState()` matches
+  toolbars by object name, so a toolbar that does not exist yet has its
+  position silently dropped.
+
+Every restore is guarded on a non-empty blob, so absent state leaves the
+`buildUi()` defaults rather than producing a zero-size window.
+
 ## 2. No way to see full message details
 
 **Observed:** From, To, Cc, Subject and the rest are not visible for the