feat(explorer): Sidebar position moves the Explorer's sidebar, the preview takes the other side (#304)

[Maxaubert]

Closes #304

What

Two Sidebar position settings (Left / Right), one per page, each moving only its own panel:

• Settings > Explorer > Layout (first row, id explorer-side, new key prism.explorer.side, default Left): "The side the places panel sits on." It moves the Explorer tab's places panel only. A project tab's tree stays where it is.
• Settings > Project settings (first row, id tree-side, key prism.tree.side, unchanged, back where it was before this PR): "The side the file tree sits on." It moves the project tab's file tree only. The Explorer stays where it is.

Find a setting finds both rows, each on its own page.

With the Explorer's Right:

• the places panel sits against the window's right edge, the preview pane against the left, the list between them;
• the preview toggle leads the address row (before Back), the end nearest the pane it opens; search stays at the far end;
• both resize grips are their left twins turned round (a drag or arrow toward the middle widens), widths are shared with Left and remembered;
• hiding the places gives the list the right edge, and resting on the right edge peeks them out over the list, shadow on the list's side; the pin keeps them;
• the places come after the list in the DOM, so Tab walks the row left to right;
• one coat of a see-through ground on every panel, as on the left.

Left is the window as it was, box for box.

Why

Owner, 2026-10-07: "fix the setting in Explorer for the sidebar where you can put it on the right side or the left side? I think that's just an empty setting for now, but actually implement it. And remember that when the sidebar goes on the right, the preview menu and button to open it would have to go on the left."

Then, after testing the first version of this PR (one shared row): "No, it should be two settings, one on the project tab and one on the explorer tab."

Decision

Two independent stores: lib/explorerSidePrefs.ts (new) for the Explorer, lib/treePrefs.ts for the project tree. App reads the Explorer's side for an Explorer tab and the tree's for a project tab (workspaceSide), and each panel peeks from its own side. Rule written into CLAUDE.md with both owner quotes. Version 0.92.1 -> 0.93.0 (feature).

Gates

• npm run typecheck: clean
• npm run lint: 0 errors (7 existing warnings)
• npx vitest run: 2529 passed, 2 skipped (171 files)
• e2e explorerSide (62 checks): both rows on their own pages with distinct subtexts; the Explorer's Right moves the places and preview and leaves the project tree exactly where it was; the project's Right moves the tree and leaves every Explorer box exactly as it was; toggle, grips, hide, peek, pin and one coat on Glacier turned round; both choices survive a restart.
• e2e, one at a time: explorerSide, panelsAlign (49), sidebarPeek (42), settingsLook (189), settingsSearch (26, incl. "Sidebar position finds the Explorer row and the project row"), sidebarPlaces (103), seeThrough (26): all ok
• npm run e2e:terminal: all 17 ok (553 checks)
• Settings screenshots of the Explorer and Project settings pages looked at, dark and light.

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https://claude.ai/code/session_01FHHaWKR4M5QtW7Wecyuk4t