feat(flutter): mobile-first theme and app shell

[Pericena]

Part A of the #109 split (theme and shell). No screen content changes.

• Puls3Breakpoints: one set of named breakpoints instead of raw 600/640/700/900/960/1000 numbers, plus isCompactLayout().
• Phone type scale below 640 px (14 px body, 18–28 px titles); desktop keeps the brand sizes.
• Compact density and denser inputs on phones; PrimaryButton is 44 px there (52 on desktop) inside a 48 px tap target.
• Bottom navigation on phones (Studio, Marketplace); the top bar keeps the logo and a wallet chip that shrinks instead of overflowing.
• ScreenHeader and SkeletonBox building blocks for the next parts.

Tests

Bottom bar on phones only, compact type and button size, no overflow on every screen at 320/360/390/414 px.

flutter analyze: 0 issues · flutter test: 94/94

[TOMOKI977]

Approved. Splitting #109 paid off: this one is focused and easy to follow. Moving every breakpoint into Puls3Breakpoints with no raw numbers left in lib, and keeping TopBar presentational while AppShell owns routing, are both nice touches.

None of these block the merge. They're worth picking up in #112 or a small follow-up:

• Puls3Text.compact is a mutable global (puls3_theme.dart:126). It is set inside Puls3App.build (app.dart:85) and read by isDense/visualDensity and the text styles. Correctness then depends on build order, and the value can leak between widget tests in the same process. Since the theme is already rebuilt from MediaQuery, I'd pass compact into the theme builder, or read it from context where it's needed.
• isCompactLayout isn't used everywhere. app.dart:86 and top_bar.dart:50 repeat width < Puls3Breakpoints.compact by hand, so changing the rule in one place would miss them. The "compact headline" rule is also duplicated in 4 places (screen_header, market_screen, studio_screen, landing_screen). A Puls3Text.headline(context) would give it a single owner.
• No tests at the thresholds. The tests cover 390 px and desktop, but not widths just below and above 640, 700, 900 or 1000, so a typo in a threshold passes CI. A small parameterized test around each boundary would cover it.

Smaller notes: the compact branch of _NavLink can't be reached because AppShell passes links: const [] on phones (top_bar.dart:66-73); on an unknown route the bottom bar selects Studio while the top bar selects nothing (app_shell.dart:36); and 600/640/700 are close enough that a one-line comment explaining why they differ would help.

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[TOMOKI977]

@Pericena merged as 1a860e8, thanks! Since it was squash-merged, #112 and #113 still carry the original theme/shell commit. Rebasing them onto main (and dropping 2dcfc9a) will keep their diffs clean when you push the fixes. The non-blocking notes from my review (the global Puls3Text.compact and the boundary tests) can go into #112 or a small follow-up.