feat(zpl): maintenance batch (^MA / ^MI / ^MW / ^JH) - #129

Adds four Setup-Script-channel commands and a dedicated Maintenance
tab in the printer-settings modal.

- ^MA (Maintenance Alert): type (H/R), print (Y/N), threshold,
  frequency, units (M/I/C). Blank slot in `print`/`units` falls
  back to spec defaults (Y / M) so partial dumps round-trip.
  frequency min=1 by deliberate parser stance; out-of-range and
  invalid type discarded.
- ^MI (Maintenance Info Message): paired with ^MA via a shared
  type discriminator. Enforced cross-field at the schema layer
  (superRefine) so a mismatched import is rejected; the UI
  cascades alert-type changes into the existing message so the
  invariant never trips silently.
- ^MW (Head Cold Warning): single Y/N flag.
- ^JH (Early Warning Settings): only f-slot (early-warning gate)
  and g-slot (head-cleaning interval, `<n>M`) modelled; sibling
  slots emit empty. The registry places ^JH BEFORE ^MA so the
  gate is in effect when the alert lands. The g-slot wire format
  is marked TODO HW-verify (Zebra docs are ambiguous between
  raw `<n>M` and an indexed 0..16 table).

UI surface:
- New `maintenance` PrinterSettingsTab + MaintenanceTab component
- Alert is fully clearable via the type select's default option;
  message text empty clears the message
- Defaults sourced from MAINTENANCE_ALERT_DEFAULTS (single source
  of truth shared by UI placeholder + parser blank-slot fallback)
- Hint shows the 63-char cap via `({MAINTENANCE_MESSAGE_MAX_LEN})`
  template, not a hardcoded string

Locales seeded English across 32 files; translators can refine.
Roadmap 98→102 / 202; READMEs Media-&-feed 8/10 and Print-quality
5/14.

Gemini PR review plus four review-iterations surfaced a class of bugs
in the printer-settings tabs: typing or pasting a char the schema
rejects (^/~/,/control) causes patchPrinterProfile to roll back, and
the controlled input visibly freezes. The fix-attempt in the original
feat commit only handled the maintenance message field and used a
regex without the global flag, so only the first unsafe char was
stripped. The same bug class already shipped in IdentityTab
(printerName, printerDescription) and EncodingAndLanguageTab
(encodingTable).

This refactor:

- Introduces `SafeStringInput` in `zplFieldPrimitives` that sanitises
  user input before patching. Default sanitiser is the new
  `stripUnsafeChars` helper, backed by a module-level g-flagged regex
  (`setupScriptUnsafeCharGlobalRegex`) so the regex isn't reallocated
  per keystroke. The primitive routes IME composition through real
  `compositionstart`/`compositionend` events instead of an
  `instanceof InputEvent` probe (cross-browser flake) and resets the
  composition flag on `blur` so a focus-loss mid-composition doesn't
  leave it stuck. Sanitises on `compositionend` as a safety net for
  browsers that skip the trailing `input` event (older Safari, some
  Android IMEs). Trade-off: the caret jumps to end after a mid-string
  strip (acceptable vs. the caret-math gymnastics the first cut
  invited).

- Hoists the cross-field cascade out of the slice into a pure
  `normalizeMaintenanceTypes(merged, patch)` helper in
  `types/PrinterProfile`. Direction follows which side the patch
  object touches, so a future `^MA`-only-via-import or a
  message-side edit each get their cascade for free.

- Migrates `printerName`, `printerDescription`, `encodingTable`, and
  the maintenance message field to `SafeStringInput`. Migrates the
  alert `print`/`units` selects and the message-text row to
  `ZplSubField` so the label-to-input `htmlFor` link is consistent
  with the rest of the modal.

- Drops the `?? 0` / `?? 1` collapse in the BoundedIntControl
  onChange handlers. `readBoundedInt` lets sub-min positives through
  for asymmetric typing; the schema requires `>= min`, so the
  frequency cell gates the commit. Threshold (`min=0`) needs no
  gate.

1528 tests green.

^MA type codes were H/R, spec is R (replacement) / C (cleaning).
^MA units default was M, spec default is I (inches).
^MA threshold/frequency caps are type-dependent per spec: R=150000m,
C=2000m, enforced via superRefine.
^JH g-slot wire format is an indexed table (0..16 → 100M..900M), not
raw <n>M with suffix as previously emitted. UI now exposes the
discrete set via a select; the persisted meters value is constrained
to the indexed set.

A v6→v7 migration remaps maintenanceAlert/Message type 'H' → 'C'
('R' was already correct). The rehydrate fixpoint loop drops any
residual schema violations top-key-by-top-key, bailing to {} if no
present key can resolve the issue.

The MaintenanceTab type-select clamps stale threshold/frequency to
the new type's cap before patching, so a switch R→C with an
out-of-spec value can't trip the schema and silently snap the
dropdown back to R.

JH_SLOT_F/G hoisted to PrinterProfile.ts so generator + parser share
one definition; JH_SLOT_COUNT replaces the hard-coded 10.