docs: always capitalize Shorebird, Flutter, and Code Push

[tomarra]

Status

READY

Description

Per the marketing content guidelines (see #576), "Shorebird", "Flutter", and "Code Push" are proper nouns and should always be capitalized. This fixes 52 instances across 16 files where they appeared lowercase or partially-cased, in prose, headings, and frontmatter description fields.

Left untouched (15 occurrences) — these are Vale false positives, not brand-name prose mentions:

• Domain names / email addresses: console.shorebird.dev, contact@shorebird.dev, download.flutter.dev
• The shorebird.yaml config filename mentioned in prose
• A GitHub issue shorthand: shorebirdtech/shorebird#3715
• A file path placeholder: /path/to/flutter
• Literal shell commands inside fenced code blocks (e.g. shorebird release android) — worth flagging to reviewers: Vale's built-in Vale.Terms rule (unlike our custom rules) does not reliably respect code-fence exclusion, so these had to be found and excluded by hand. Something to watch for if this check gets automated/blocking later.

Judgment calls worth a second look during review:

• A few instances use "code push" as a generic industry term rather than Shorebird's specific feature name (e.g. "there are multiple other Code Push services publicly available", referring to competitors). I capitalized these per the literal guideline, but marketing may want "code push" lowercase when it's the generic technology category vs. Shorebird's own product.
• "run Shorebird init" / "our Shorebird preview output" (in percentage-based-rollouts.mdx) — capitalizing the brand name reads slightly unusually right before a CLI subcommand name. Flagging in case marketing prefers a different phrasing here.

This is one of several small content PRs split out of #576 so the Vale.Terms check can eventually be made blocking without a giant, hard-to-review PR.

Test plan

• npm run build succeeds, no broken links
• cspell passes on all touched files
• Verified locally with the (not-yet-merged) Vale config from docs: add style checks for headings and content guidelines #576 — exactly the expected 15 known-false-positive occurrences remain flagged, everything else resolved