feat: add threat model comparison matrix

[Nacho1499]

Closes #129

PR Description:
Implements the standalone Threat Model comparison matrix page for prospective integrators, detailing what various threat actors can and cannot learn across different privacy approaches.

Type of Change:
-New feature

Scope of Changes:
-Added a standalone threat model route and page for the comparison matrix.
-Added structured threat-model data covering the relevant threat actors and privacy approaches.
-assumption footnotes for every matrix cell.
-Added reviewer credit and opt-in confirmation.
-Linked the page from the Security and Privacy flows.
-Ensured the table remains mobile-responsive with horizontal scrolling.

How Has This Been Tested:
-Route-level regression test added and passed
-Production build verified
-Lighthouse performance and SEO checks run and confirmed at or above 90

Checklist:

• I have performed a self-review of the change
• The change is scoped to the requested feature
• No secrets or sensitive data are included in this PR
• The branch is up to date with the target branch

Verification notes:
-Route test: pnpm exec vitest run threat-model.test.tsx
-Build: pnpm build:

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[truthixify]

Merged @Nacho1499. Footnoted assumptions per cell is what keeps this a threat model rather than a marketing table. Note the acceptance item about cryptographer sign-off on the content is still open; the structure is in, and I will arrange a reviewer pass before this gets promoted anywhere prominent.