fix: allow the main window to be resized horizontally

[git-lucky]

Why

The main window is locked to 450pt wide and can't be widened, while it resizes vertically without complaint. Transcriptions are often long, so the narrow column makes the history list harder to read than it needs to be.

Root cause

Three independent constraints pinned the width. All three had to go — fixing any one or two alone changes nothing, which is what made this less obvious than it looks:

1. SwiftUI — .frame(width: 450) on the root view set a fixed width, making the content's minimum and maximum identical. .windowResizability(.contentMinSize) then had nothing to resize.
2. AppKit properties — adoptMainWindow set window.maxSize width to 450, equal to minSize.width.
3. AppKit delegate — windowWillResize(_:to:) returned NSSize(width: 450, height: frameSize.height), rewriting the width on every resize attempt.

The third was decisive: AppKit consults the delegate on each resize, and its return value overrides the window's own minSize/maxSize. It also explains the exact symptom — height was passed through untouched while width was hardcoded, so the window resized vertically but never horizontally.

What changed

	Before	After
SwiftUI frame	.frame(width: 450)	.frame(minWidth: 450)
window.maxSize.width	450	unbounded
windowWillResize	forced width to 450	returns the proposed size
.defaultSize	450 × 650	unchanged
Height range	400–900	unchanged

ContentView already declared .frame(minWidth: 400, idealWidth: 400), so the content was always built to flex — only the outer constraints prevented it.

Existing users see no difference on update. The window still opens at 450 × 650; it simply can now be widened. SwiftUI persists the frame automatically, so a user who never drags it keeps exactly today's behavior.

Tests

5 new unit tests in MainWindowResizabilityTests. The size limits are extracted into AppDelegate.applyMainWindowSizeLimits(to:) so they can be applied to a real NSWindow and asserted, following the pattern already used by isMainAppWindow:

• testWindowWillResize_honorsProposedWidth — fails against the old code, passes against the new. This is the real regression guard.
• testWindowWillResize_leavesHeightUntouched
• testSizeLimits_allowHorizontalResizing — max width must exceed min width
• testSizeLimits_preserveMinimumWidth — 450 floor retained
• testSizeLimits_preserveHeightRange — 400–900 retained

Test plan

• Window opens at 450 × 650 as before
• Right and left edges now drag wider
• Cannot be dragged narrower than 450pt
• Vertical resizing still honors the 400–900 range
• Size persists across relaunch (SwiftUI frame autosave)