Refactor avatar package, support default avatar fallback

[wxiaoguang]

• Fix Avatars broken (404) after turning on offline mode #34715

[Copilot]

Pull request overview

This PR fixes issue #34715 where avatars return 404 errors when offline mode is enabled (because Gravatar URLs can't be fetched). The core fix is to return a randomly-generated identicon image instead of a 404 when a requested avatar file doesn't exist in storage. The PR also refactors the avatar serving infrastructure, simplifies the identicon.New() and RandomImage*() APIs by removing error returns, and consolidates HTTP cache helper functions into clearer public/private variants.

Changes:

• When a user/repo avatar is not found in storage, instead of returning HTTP 404, the server now generates and serves a deterministic random identicon (seeded by the avatar path) with proper public cache-control headers.
• The HandleGenericETagCache / HandleGenericETagTimeCache functions are replaced by HandleGenericETagPublicCache / HandleGenericETagPrivateCache, which now always require a lastModified *time.Time parameter, and are used consistently across file/LFS/attachment serving endpoints.
• The RandomImageSize, RandomImage, and identicon.New() APIs are simplified to never return errors (the identicon package now clamps size values instead of returning an error for out-of-range inputs).

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
routers/web/base.go	Core fix: serves a fallback random avatar instead of 404 when avatar is not found
modules/avatar/avatar.go	Simplifies RandomImageSize/RandomImage to return image.Image (no error)
modules/avatar/identicon/identicon.go	New() no longer returns error; clamps size; uses default dark colors if none provided
modules/avatar/avatar_test.go	Removes old error-checking tests; adds benchmark
modules/httpcache/httpcache.go	Removes HandleGenericETagCache; adds HandleGenericETagPublicCache and HandleGenericETagPrivateCache
modules/httpcache/httpcache_test.go	Rewrites test to use table-driven approach for new private cache function
modules/assetfs/embed.go	Exports GetExecutableModTime for use in avatar fallback handler
models/user/avatar.go	Simplifies error handling with new infallible RandomImage API
models/repo/avatar.go	Simplifies error handling; fixes error message capitalization
routers/common/serve.go	Uses HandleGenericETagPrivateCache instead of removed function
routers/web/repo/download.go	Uses HandleGenericETagPrivateCache for blob/LFS caching
routers/web/repo/attachment.go	Uses HandleGenericETagPrivateCache for attachment caching
routers/api/v1/repo/file.go	Uses HandleGenericETagPrivateCache for raw file/LFS API caching
tests/integration/user_avatar_test.go	Extends test to verify avatar content, redirect behavior, and the new fallback response

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[Copilot]

Pull request overview

Copilot reviewed 14 out of 14 changed files in this pull request and generated 1 comment.

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