docs: refresh docs and align versions for v1.1.0

[OrF8]

Motivation

• Prepare the repository for the v1.1.0 release by bringing all public-facing documentation into sync with the current code and feature set.
• Remove or replace stale/outdated guidance and ensure security/reporting guidance is realistic for the project.
• Align package metadata so release artifacts and docs reference a consistent version line.

Description

• Updated README.md to accurately describe the current app (boards, one-level super-board hierarchy, direct vs inherited membership, invite flow, transaction create/edit/delete, negative amounts, future-dated transaction dates, and Excel export including super-board multi-sheet + summary behavior), developer setup, env variables, and deployment workflows.
• Rewrote SECURITY.md with a supported-versions table (marking 1.1.x supported), private reporting guidance via GitHub security workflows, realistic disclosure expectations, and scope notes.
• Added CHANGELOG.md (Keep-a-Changelog style) with Unreleased, 1.1.0, and minimal historical 1.0.x entries describing user-facing changes.
• Bumped release version to 1.1.0 in package.json and functions/package.json and updated lockfiles (package-lock.json, functions/package-lock.json) to reflect the version alignment; no application behavior changes were introduced.

Testing

• Ran npm run build which completed successfully (production build via Vite).
• Ran npm run lint which surfaced pre-existing frontend ESLint errors (the lint step failed; these issues were not introduced by this docs/metadata change).
• Ran npm --prefix functions run lint which intentionally skips lint and completed (functions lint script is a placeholder).

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Codex Task

[Copilot]

Pull request overview

This PR prepares the repo for the v1.1.0 release by bringing public-facing documentation and package metadata into alignment with the current feature set and release versioning.

Changes:

• Refreshed README.md to reflect current functionality (boards/super-boards, membership model, invite flow, transactions, export) and updated dev/deploy guidance.
• Reworked SECURITY.md to clarify supported versions (1.1.x) and establish private vulnerability reporting guidance.
• Added a CHANGELOG.md and aligned package versions/lockfiles to 1.1.0.

Reviewed changes

Copilot reviewed 5 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
SECURITY.md	Updates supported-versions policy and private vulnerability reporting instructions.
README.md	Rewrites product + developer documentation to match current architecture, features, and workflows.
CHANGELOG.md	Introduces Keep-a-Changelog-style release notes including 1.1.0.
package.json	Bumps frontend package version to 1.1.0.
package-lock.json	Aligns lockfile metadata with 1.1.0 (and reflects lockfile regeneration).
functions/package.json	Bumps functions package version to 1.1.0.
functions/package-lock.json	Aligns functions lockfile metadata with 1.1.0.

Files not reviewed (1)

• functions/package-lock.json: Language not supported

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[OrF8]

#67 Will do the same but better.