refactor(design): lock the border-radius ladder, one radius per role (#1607)

Seven radii were in circulation (4/5/6/8/12/16px + pill) with no rule for
which went where; one toolbar row on /transactions mixed four shape
languages. This locks a 4-tier ladder (design.md convention 16):

- pill: interactive toolbar controls (buttons, chips, pickers, segmented
  controls, toolbar search, count nubs)
- rounded-xl (12px): overlay tier: page panel, dialogs, slide-overs
- rounded-lg (8px): cards, form fields, popover/menu content, boxes
- rounded-sm (4px): nested leaves (menu items, checkboxes, kbd/code nubs)

Changes:
- New SegmentedControl primitive (pill-in-pill tablist, h-8) replaces the
  hand-rolled bg-muted/70 tablist copied across 11 files
- New ToolbarSearch primitive (pill, h-8) adopted on 9 page toolbars;
  dialog/picker searches keep the rounded-lg Input
- dialog.tsx 8px -> 12px, matching SettingsModal/slide-over/CommandPalette
- ContextPicker chips at the shared h-8 toolbar height
- ~300 rounded-md / bare rounded call sites remapped by role; auth icon
  tiles and the mobile nav sheet come down from 16px to 12px
- rounded-md, bare rounded, rounded-2xl and rounded-[Npx] are dead
  vocabulary, enforced by a new off-ladder-radius check in check:guards

Verified: lint 0 errors, 14422 unit tests pass, check:guards green, tsc
clean on all changed files, sandbox screenshots of transactions/
bookkeeping/granskning toolbars and the Ny verifikation dialog.

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

components/settings/BackupDownloadForm.tsx

@@ -277,7 +277,7 @@ export function BackupDownloadForm() {
                 id="backup-period"
                 value={selectedPeriodId}
                 onChange={(e) => setSelectedPeriodId(e.target.value)}
-                className="flex h-10 w-full max-w-xs rounded-md border border-input bg-background px-3 py-2 text-sm ring-offset-background focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2"
+                className="flex h-10 w-full max-w-xs rounded-lg border border-input bg-background px-3 py-2 text-sm ring-offset-background focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2"
                 disabled={periods === null || periods.length === 0}
               >
                 {/* The confirmed-empty option only after a confirmed empty
@@ -308,7 +308,7 @@ export function BackupDownloadForm() {
             />
           </div>
 
-          <div className="rounded-md border border-border bg-muted/30 p-3 text-sm">
+          <div className="rounded-lg border border-border bg-muted/30 p-3 text-sm">
             <div className="flex items-center gap-2 text-muted-foreground">
               <Info className="h-3.5 w-3.5" />
               {isLoadingEstimate ? (