refactor(web): rewrite UX copy for clarity and consistency

[driaug]

The copy across the dashboard was written screen by screen, so the same concept was phrased several different ways and a lot of text explained things the UI already said. This is a pass over every user-facing string in apps/web against one standard, applied consistently.

Systemic patterns removed:

• 25 confirmation dialogs opened with "Are you sure you want to...", then repeated the question the title already asked. Titles now name the object ("Delete john@acme.com?"), bodies state only the consequence, and buttons are verb + object. Cancel dialogs previously rendered [Cancel] [Cancel Campaign]; the dismiss button now reads "Keep sending".
• 42 success toasts ended in "successfully". The toast is already green.
• 87 "Failed to X" strings gave no cause and no next step. Most sites already surfaced error.message and only used these as a fallback, so the fallbacks now name what did not happen and what to do.
• ~20 CardDescriptions restated their own CardTitle. Deleted. The ones carrying an actual rule were kept and sharpened instead.
• BillingConsumption, BillingInvoices and QuickStart each wrote their header 3-4 times across loading/error/empty branches. Hoisted to one header per component.
• 243 Title Case UI strings across 190 distinct labels moved to sentence case. The app previously shipped both "Send To" and "Send to".
• Required-field markers used three different mechanisms. Now one.
• Terminology: "API Credentials" and "API Keys" were the same card under two names; cancelled/canceled and .../… were both in use; the templates page title disagreed with its nav item.

Structural changes:

• Removed the analytics "Performance Insights" card. It dispensed advice the product cannot substantiate ("above industry average" against no cited benchmark, "add more compelling calls-to-action") in a product positioned against exactly that tone. Replaced with two counters drawn from real data: events triggered and workflow runs started.
• Subscription activation copy was ~75 words over two paragraphs. It is load-bearing (it prevents billing surprises), so it was restructured rather than cut: one summary line plus a "Why two charges?" disclosure.
• Consolidated the DKIM instructions that appeared both above and below the DNS record table.
• Trimmed the workflow active-executions notice and the HTML-to-visual editor warning to their load-bearing lists.

Deliberately untouched:

• packages/shared/src/i18n/locales/* and the public subscribe, unsubscribe and manage pages. Their copy is translated into 16 locales and editing English there would desync 15 of them.
• The onboarding flows and the dashboard greeting, whose copy is already the voice everything else was moved toward.
• console.error strings, which are developer-facing.

PRODUCT.md replaces the legacy .impeccable.md, with the register made explicit and a terminology section recording that user-facing nouns (Contact, Event, Campaign, Workflow, Template, Segment) track the API.

Typecheck and production build pass.