fix(desktop): in-app browser kept squishing the chat and jamming its resize sash

[OutThisLife]

Reported on Discord: with the built-in browser open, the conversation column keeps getting crushed ("my chat ends up 3 characters wide") and dragging the browser's left edge only moves a little per attempt.

Three bugs in the pane shell lined up to produce that. The browser is an Electron <webview>, which hit-tests in its own process — a sash drag went silent as soon as the pointer crossed into the page, so shrinking the browser barely worked while growing it worked fine. A multi-tab flex zone lost its panes' min-width floors, so a chat zone with session tabs stacked in could be dragged down to the generic 80px floor and persisted there. And every close/reopen of the browser re-split the chat zone [1, 1], taking half the conversation again no matter what size the user had left it at.

Summary

• New guardGuestPointers() makes webview/iframe guests transparent to hit-testing for the life of a drag (sash resize, preview console resize, tab drag), so the gesture follows the pointer across the browser; the tree sash also gains blur/lostpointercapture teardown like the console sash already had.
• sizingFor now aggregates min floors for flex stacks too (largest-tenant, same rule as fixed stacks); caps still apply only where they speak for the whole zone.
• A tile pane that leaves the tree alone in its zone records the share it held against its seam neighbor (hermes.desktop.paneShare.v1); adoption and dockPaneBeside re-insert with that share instead of an even split.

Test plan

• npx vitest run src/components/pane-shell — 103 tests, including new pane-share-memory.test.ts (first open splits evenly, reopen restores the recorded share, stacked tabs record nothing)
• tsc --noEmit clean
• In the app: drag the browser's left seam rightward across the page — it tracks the pointer the whole way
• Shrink the browser, close the tab, have the agent open a URL — the browser comes back at the size it was closed at
• With two session tabs open, confirm the chat column can't be dragged below its min width

[github-actions]

૮ >ﻌ< ა ci review

_{running on 33ac4bc — fix(desktop): satisfy eslint on pane-share-memory test}

_{waiting for jobs to start…}