fix: position window beside tray on left taskbars

[afonsojramos]

Summary

When the OS taskbar is anchored to the left edge of the screen, getWindowPosition returned 'bottomLeft', which puts the menubar window in the screen's bottom-left corner — visually disconnected from the tray icon. This PR switches that branch to 'leftCenter' so the window sits vertically centered against the left work-area edge, much closer to where the tray icon actually lives.

Why this design over upstream's

The motivating bug was reported in upstream max-mapper/menubar#293 by @thomasdarde, who patched the (then-current) 'trayBottomLeft' to 'center'. Their own commit message says it best: "It's not perfect, but at least not broken."

The fork has since moved the value to 'bottomLeft' (which avoids clipping under the taskbar — bottomLeft reads from workArea.x, not tray.x), but the window still pops up far from the tray.

Option	Clips under taskbar?	Close to tray?
trayBottomLeft (old upstream)	✅ Yes, x reads from tray.x ≈ 0	—
bottomLeft (current main)	❌ No	❌ No, window appears at screen bottom
center (upstream max-mapper#293)	❌ No	❌ No, window appears mid-screen
leftCenter (this PR)	❌ No	✅ Vertically centered on the left edge — same strip as the tray

leftCenter resolves to { x: workArea.x, y: workArea.height / 2 - windowHeight / 2 } in Positioner, so the left edge sits flush against the work area (no clipping) and the window is vertically anchored to the centre of the strip where the tray lives.

This isn't pixel-perfect tray anchoring — Positioner doesn't have a side-taskbar tray-anchored option — but it's strictly better than the alternatives.

Test plan

• bun run test — 62/62 pass
• bun run typecheck — clean
• bun run lint:check — clean (one pre-existing warning, unrelated)
• Manual: would benefit from real-world verification on a left-side Windows taskbar. The CI tray-rendering matrix covers Linux DEs but not (yet) left-anchored variants.

Closes the gap from upstream max-mapper/menubar#293.