Android: swipe from the left edge for chat history

[Gratenes]

Stacked on #609; I'll retarget to main once that lands.

Getting between chats on Android right now means backing out to Sessions, scrolling, and tapping back in. This adds a history drawer so you can jump between chats, or start a new one, from wherever you are.

Swipe right from the left edge (or tap the new menu button in a chat's header) and the drawer slides in under your finger:

• New chat at the top
• A project picker to narrow the list to one project
• All / Needs you / Working / Pinned filters, as one segmented control with a sliding pill
• Recent chats, newest first, each with a status dot (working, needs you, failed), the project badge and how long ago it was active. The chat you're in is highlighted.
• Settings at the bottom

Chat header	Drawer open

Notes

• In a chat, the header's back button becomes the menu button. Back still works through the system back gesture.
• With gesture navigation, a swipe from the left edge normally means back, and Android only lets an app reserve about 200dp of that edge. The drawer reserves the middle band, where the grip is, so swipes above or below it still go back. The menu button always works.
• The Sessions page filters now use the same segmented control as the drawer, so the two match (counts stay, inside the track). The old Pill component is removed since nothing else used it.
• Tablets and unfolded foldables are unchanged, since they already show the session list next to the chat.
• Kotlin/Compose only, no changes to the Rust core. It reuses the same session rows as the Sessions screen.

Testing

Checked on an emulator with the demo workspace: edge swipe, the menu button in a chat, tap outside to close, picking a chat, and the filters in both the drawer and on the Sessions page.

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