Two-column layout: replace calc()-based heights with a viewport-locked flex shell

Background

musehub#84 fixed the original bug (sticky sidebars clipped and couldn't scroll) by moving from position: sticky to a bounded-height pattern: each column got max-height: calc(100dvh - var(--sticky-offset, var(--header-height)) - ...) plus overflow-y: auto.

That pattern has now broken twice, both for the same underlying reason — --sticky-offset is a static CSS guess:

body:has(.repo-tabs) {
  --sticky-offset: calc(var(--header-height) + var(--repo-tabs-height));
}

It only accounts for the navbar + repo-tabs bar. Any page with additional chrome between the header and the two-column grid (a toolbar, filter bar, hero section, stat strip) has a wrong --sticky-offset, so every column's computed max-height is off by however tall that extra chrome is:

1. Footer-overlap regression (fixed this cycle) — the calc() put on the grid wrapper itself (an earlier iteration of this pattern) claimed a full viewport's height measured from wherever the grid happened to sit, pushing the footer down to overlap still-visible content on pages with a hero/eyebrow above the grid.
2. Unreachable pagination regression (this ticket) — on the issues list page, .isl-main's max-height doesn't account for the issues-list toolbar above .isl-layout, so the column's box extends past the visible viewport with no way to scroll to the pagination controls at the bottom.

Patching the formula a third time (adding yet another term to --sticky-offset, or a per-page override) just sets up the next page that adds chrome above its grid to hit the same bug again. The pattern itself — hand-deriving "how much vertical space is left" per page in CSS — is the defect, not any single formula.

Goal

Replace the calc()-based height budget with a viewport-locked flex shell, the same pattern Gmail/Slack/VS Code/Notion use for independently-scrollable sibling panes: the shell claims height: 100dvh once, every fixed-height sibling above the scrollable region is flex-shrink: 0, and the scrollable region itself is flex: 1; min-height: 0. The browser computes "whatever's left" automatically — no calc(), no magic numbers, no per-page offset to get wrong, and this bug class becomes structurally impossible rather than something to keep re-patching.

Confirmed with gabriel: Gmail's own layout (header fixed, no footer while inside the app view, two panes each scroll independently within a viewport-locked shell) is the reference UX to match.

Design

• base.html gets an opt-in body class, body-app-shell, set only by templates that use a two-column layout. Pages without one (home, plain forms, docs/marketing pages) are untouched — normal document scroll, footer visible.
• body.body-app-shell: height: 100dvh; overflow: hidden; display: flex; flex-direction: column;. Navbar and repo-tabs: flex-shrink: 0. #content: flex: 1; min-height: 0; overflow: hidden; display: flex; flex-direction: column;.
• The footer is hidden on body-app-shell pages — matches Gmail; a marketing-site footer doesn't belong on a dense two-column app screen. Pages that keep normal scroll keep the footer exactly as today.
• Every wrapper between #content and the actual two-column grid (per-page toolbars, etc.) becomes flex: 1; min-height: 0; display: flex; flex-direction: column; so the "fill remaining space" chain isn't broken partway down.
• Each column (*-main, *-sidebar) keeps min-height: 0; overflow-y: auto; — no max-height, no calc().
• --sticky-offset / --repo-tabs-height are deleted once nothing references them.

Affected pages (11)

• [ ] Repo home (_repo-home.scss / repo_home.html)
• [ ] Issues list (_issues.scss isl-* / issue_list.html) — the page that's actually broken right now; verify this one first.
• [ ] Issue detail (_issues.scss isd-* / issue_detail.html)
• [ ] Proposal detail (_proposal-detail.scss / proposal_detail.html)
• [ ] Explore/browse (_explore.scss / explore.html)
• [ ] Docs (_docs.scss / docs.html)
• [ ] Muse dev docs (_muse-docs.scss / muse_docs.html or equivalent)
• [ ] Blame (_blame.scss / blame.html)
• [ ] Blob/outline panel (_blob.scss — different pattern, verify it still works once the shell change lands, may not need edits)
• [ ] Profile graph sidebar (_profile.scss .graph-sidebar / profile.html)
• [ ] _layout.scss's shared .layout-two-col/.layout-sidebar utility classes (whichever pages actually use them, if any still do)

Each page verified individually (screenshot or live check) before moving to the next, then merged together once all are converted and the sticky-offset machinery is fully removed.

Coordination note

aaronrene picked up musehub#129 (rich social preview cards / OG images) at the same time this work started. That ticket touches musehub/api/routes/musehub/_ui_helpers.py, ui_repo.py (backend route, OG tag wiring), and a new /opengraph-image endpoint — no overlap expected with this ticket's scope (base.html, SCSS, page templates' layout markup). Flagging here so neither of us has to guess.